United Nations Secretary General Ban Ki-moon stands by his window as he poses for a portrait in his office at United Nations Headquarters in the Manhattan borough of New York, New York, U.S., October 21, 2016. REUTERS/Carlo Allegri

United Nations Secretary General Ban Ki-moon sits at his desk as he poses for a portrait in his office at United Nations Headquarters in the Manhattan borough of New York, New York, U.S., October 21, 2016. REUTERS/Carlo Allegri

United Nations Secretary General Ban Ki-moon sits at his desk as he poses for a portrait in his office at United Nations Headquarters in the Manhattan borough of New York, New York, U.S., October 21, 2016. REUTERS/Carlo Allegri

United Nations Secretary General Ban Ki-moon looks out his window as he poses for a portrait in his office at United Nations Headquarters in the Manhattan borough of New York, New York, U.S., October 21, 2016. REUTERS/Carlo Allegri

United Nations Secretary General Ban Ki-Moon is seen during an interview with Reuters at U.N. headquarters in New York, September 14, 2010. REUTERS/Mike Segar/File Photo

U.N. Secretary-General Ban Ki-moon speaks to the press at the United Nations European headquarters in Geneva, Switzerland October 3, 2016. REUTERS/Pierre Albouy

U.N. Secretary-General Ban Ki-moon empty desk is seen before his speech to the press conference at the United Nations European headquarters in Geneva, Switzerland October 3, 2016. REUTERS/Pierre Albouy

United Nations Secretary General Ban Ki-moon looks up during a tribute to the late King of Thailand Bhumibol Adulyadej in the General Assembly at United Nations headquarters in New York, October 28, 2016. REUTERS/Brendan McDermid

United Nations Secretary General Ban Ki Moon presides over the starting session of the United Nations General Assembly in the Manhattan borough of New York, U.S., September 20, 2016. REUTERS/Mike Segar

United Nations Secretary General Ban Ki-moon bows towards those gathered for the General Debate of the 71st Session of the United Nations General Assembly in the Manhattan borough of New York, U.S., September 20, 2016. REUTERS/Mike Segar

United Nations Secretary General Ban Ki-moon addresses the General Debate of the 71st Session of the United Nations General Assembly in the Manhattan borough of New York, U.S., September 20, 2016. REUTERS/Mike Segar

United Nations Secretary General Ban Ki-moon holds up a smart phone as he addresses the General Debate of the 71st Session of the United Nations General Assembly in the Manhattan borough of New York, U.S., September 20, 2016. REUTERS/Mike Segar

United Nations Secretary General Ban Ki-moon addresses the General Debate of the 71st Session of the United Nations General Assembly in the Manhattan borough of New York, U.S., September 20, 2016. REUTERS/Mike Segar

U.S. President Barack Obama shakes hands with U.N. Secretary General Ban Ki-moon after addressing the United Nations General Assembly in the Manhattan borough of New York, U.S., September 20, 2016. REUTERS/Mike Segar

U.S. President Barack Obama shakes hands with the President of the seventy-first session of the United Nations General Assembly Peter Thomson and with U.N. Secretary General Ban Ki Moon after addressing the United Nations General Assembly in the Manhattan borough of New York, U.S., September 20, 2016. REUTERS/Lucas Jackson

U.S. President Barack Obama shakes hands with UN Secretary General Ban Ki-moon following his address to the United Nations General Assembly in New York September 20, 2016. REUTERS/Kevin Lamarque

U.S. President Barack Obama shakes hands with U.N. Secretary General Ban Ki Moon (R) after addressing the United Nations General Assembly in the Manhattan borough of New York, U.S., September 20, 2016. REUTERS/Lucas Jackson

U.S. President Barack Obama shakes hands with the President of the seventy-first session of the United Nations General Assembly Peter Thomson and with U.N. Secretary General Ban Ki-moon (L) after addressing the United Nations General Assembly in the Manhattan borough of New York, U.S., September 20, 2016. REUTERS/Carlo Allegri TPX IMAGES OF THE DAY

U.S. President Barack Obama and UN Secretary General Ban Ki-moon share a toast at a luncheon during the United Nations General Assembly at United Nations headquarters in New York City, U.S. September 20, 2016. REUTERS/Lucas Jackson

U.S. President Barack Obama and UN Secretary General Ban Ki-moon share a toast at a luncheon during the United Nations General Assembly at United Nations headquarters in New York City, U.S. September 20, 2016. REUTERS/Lucas Jackson

U.S. President Barack Obama laughs as UN Secretary General Ban Ki-moon mentions playing golf after their terms are up at a luncheon during the United Nations General Assembly at United Nations headquarters in New York City, U.S. September 20, 2016. REUTERS/Lucas Jackson

U.S. President Barack Obama toasts as UN Secretary General Ban Ki-moon listens at a luncheon during the United Nations General Assembly at United Nations headquarters in New York City, U.S. September 20, 2016. REUTERS/Lucas Jackson

U.S. President Barack Obama toasts as UN Secretary General Ban Ki-moon listens at a luncheon during the United Nations General Assembly at United Nations headquarters in New York City, U.S. September 20, 2016. REUTERS/Lucas Jackson

U.S. President Barack Obama listens as UN Secretary General Ban Ki-moon toasts at a luncheon during the United Nations General Assembly at United Nations headquarters in New York City, U.S. September 20, 2016. REUTERS/Lucas Jackson

Japan's Prime Minister Shinzo Abe (L) arrives with United Nations Secretary General Ban Ki-Moon on the sidelines of the United Nations General Assembly at United Nations headquarters in New York City, U.S. September 20, 2016. REUTERS/Brendan McDermid

United Nations Secretary General Ban Ki-moon holds a joint news conference with Swedish Prime Minister Stefan Lofven (not pictured) at the Swedish Government headquarters Rosenbad in Stockholm, Sweden, March 30, 2016. REUTERS/Maja Suslin/TT News Agency ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS PICTURE IS DISTRIBUTED EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS. SWEDEN OUT. NO COMMERCIAL OR EDITORIAL SALES IN SWEDEN. NO COMMERCIAL SALES.

U.N. Secretary-General Ban Ki-moon arrives for the Conference on the Prevention of Violent Extremism at the United Nations in Geneva, Switzerland, April 8, 2016. REUTERS/Denis Balibouse

U.N. Secretary-General Ban Ki-moon addresses the Conference on the Prevention of Violent Extremism at the United Nations in Geneva, Switzerland, April 8, 2016. REUTERS/Denis Balibouse

U.N. Secretary-General Ban Ki-moon gestures during a news conference at the Conference on the Prevention of Violent Extremism at the United Nations in Geneva, Switzerland, April 8, 2016. REUTERS/Denis Balibouse

Ban Ki-moon (R), Secretary-General of the United Nations, chats with French President Francois Hollande before delivering his opening remarks at the Paris Agreement signing ceremony on climate change held at the United Nations Headquarters in Manhattan, New York, U.S., April 22, 2016. REUTERS/Mike Segar

United Nations Secretary-General Ban Ki-moon delivers his opening remarks at the Paris Agreement signing ceremony on climate change at the United Nations Headquarters in Manhattan, New York, U.S., April 22, 2016. REUTERS/Brendan McDermid

Ban Ki-moon, Secretary-General of the United Nations, delivers his opening remarks at the Paris Agreement signing ceremony on climate change as French President Francois Hollande (top, 2nd from L) looks on at the United Nations Headquarters in Manhattan, New York, U.S., April 22, 2016. REUTERS/Mike Segar TPX IMAGES OF THE DAY

United Nations Secretary-General Ban Ki-moon delivers his opening remarks at the Paris Agreement signing ceremony on climate change at the United Nations Headquarters in Manhattan, New York, U.S., April 22, 2016. REUTERS/Brendan McDermid

Palestinian President Mahmoud Abbas shakes hands with U.N. Secretary General Ban Ki-moon (R) after signing the Paris Agreement on climate change at the United Nations Headquarters in Manhattan, New York, U.S., April 22, 2016. REUTERS/Mike Segar

United Nations Secretary-General Ban Ki-moon (R) and French President Francois Hollande hold a press conference on the sidelines of the Paris Agreement on climate change held at the United Nations Headquarters in Manhattan, New York, U.S., April 22, 2016. REUTERS/Brendan McDermid

French President Francois Hollande (2nd from L) listens as he holds a press conference with United Nations Secretary-General Ban Ki-moon as French Minister of Ecology, Sustainable Development and Energy Segolene Royal (L) and Costa Rican diplomat Christiana Figueres listen on the sidelines of the Paris Agreement on climate change at the United Nations Headquarters in Manhattan, New York, U.S., April 22, 2016. REUTERS/Brendan McDermid

U.N. Secretary General Ban Ki-moon (C) embraces U.S. Secretary of State John Kerry (R) who holds his two-year-old granddaughter Isabelle Dobbs-Higginson after signing the Paris Agreement on climate change at United Nations Headquarters in Manhattan, New York, U.S., April 22, 2016. REUTERS/Carlo Allegri TPX IMAGES OF THE DAY

U.N. Secretary-General Ban Ki-moon arrives for the closing news conference during the World Humanitarian Summit in Istanbul, Turkey, May 24, 2016. REUTERS/Murad Sezer

UN Secretary-General Ban Ki-moon gestures as he speaks during the panel "The New Climate and Development Imperative" at the World Economic Forum in Davos, Switzerland, Thursday, Jan. 21, 2016. World leaders are holding a flurry of diplomatic meetings at the World Economic Forum and worried CEOs are debating about how to deal with this year's volatile markets and low oil prices. (AP Photo/Michel Euler)

United Nations Secretary-General Ban Ki-moon speaks during the launch of the UNâs report on humanitarian financing in Dubai, the United Arab Emirates, Sunday, Jan 17, 2016. (AP Photo/Martin Dokoupil)

United Nations Secretary-General Ban Ki-moon prepares to speak during the launch of the UNâs report on humanitarian financing in Dubai, the United Arab Emirates, Sunday, Jan 17, 2016. (AP Photo/Martin Dokoupil)

United Nations Secretary-General Ban Ki-moon speaks during the launch of the UNâs report on humanitarian financing in Dubai, the United Arab Emirates, Sunday, Jan 17, 2016. (AP Photo/Martin Dokoupil)

United Nations Secretary-General Ban Ki-moon arrives to a press conference ahead of the launch of the UNâs report on humanitarian financing, in Dubai, United Arab Emirates, Sunday, Jan 17, 2016. (AP Photo/Martin Dokoupil)

United Nations Secretary-General Ban Ki-moon speaks at a press conference ahead of the launch of the UNâs report on humanitarian financing, in Dubai, United Arab Emirates, Sunday, Jan 17, 2016. (AP Photo/Martin Dokoupil)

United Nations Secretary-General Ban Ki-moon speaks to reporters before a Security Council meeting at U.N. headquarters, Wednesday, Jan. 6, 2016. North Korea trumpeted its first hydrogen bomb test Wednesday, a powerful, self-proclaimed "H-bomb of justice" that would mark a major and unanticipated advance for its still-limited nuclear arsenal. Pyongyang's announcement was met with widespread skepticism, but whatever the North detonated in its fourth nuclear test, another round of tough international sanctions looms for the defiant, impoverished country. (AP Photo/Seth Wenig)

United Nations Secretary-General Ban Ki-moon addresses the meeting on global responsibility sharing through pathways for admission of Syrian refugees, at the United Nations in Geneva, Switzerland, March 30, 2016. REUTERS/Denis Balibouse

United Nations Secretary-General Ban Ki-moon adjusts a sign before the meeting on global responsibility sharing through pathways for admission of Syrian refugees, at the United Nations in Geneva, Switzerland, March 30, 2016. REUTERS/Denis Balibouse